Machine tool with workpiece mass measurement

Abstract

Machine tool (10), in particular machine tool with static compensation of the workpiece weight, with a machine frame (13) for stationary installation, with at least one workpiece holding device (12) which is movably mounted with respect to the machine frame (13) with at least one first degree of freedom, with at least one drive device (16) connected to the workpiece holding device (12) to move it in at least one direction (Z) determined by the first degree of freedom, with at least one detection device (15) which is configured to detect a signal (S z ) to generate the force (F) exerted by the drive device (16) on the workpiece holding device (12). z ) indicates, with an evaluation unit (23) which receives the signal (S z ) is supplied to extract the signal (S z) and the movement performed by the workpiece holding device (12) to determine at least the mass (m) of the workpiece (11), characterized by that the first degree of freedom allows a horizontal linear movement to move the workpiece holding device (12) linearly in at least one direction (Z) determined by the first degree of freedom, and that the executed movement is an accelerated movement, whereby the workpiece mass (m) is determined by the detected force (F) z ) and the measured acceleration (z'') is calculated.

Description

[0001] The invention relates to a machine tool with a device for detecting the workpiece mass. In particular, the invention relates to a machine tool with a device for determining the workpiece mass and the spatial position of the workpiece's center of gravity.

[0002] DE 10 2009 056 492 B4 discloses a gantry-style machining center with a balancing device for the workpiece table. The machine frame is U-shaped and has two side walls and a bridge support mounted on the side walls, which can pivot about a horizontal axis. A driven, rotatable workpiece table with at least one measuring system for determining its rotational speed and angular position is provided in the bridge support. A clamping surface for receiving and securing workpieces for milling, drilling, or turning is provided on the top of the table. A measuring device for determining table imbalance is provided in the bridge support. Furthermore, at least two balancing weights are provided on the workpiece table to compensate for table imbalance. A control and regulating device is provided for determining the position of the balancing weights on the workpiece table.

[0003] Furthermore, EP 2 735 928 A1 discloses a machine tool whose control establishes a relationship between workpiece mass, workpiece center of gravity, and elastic component deformation. This relationship is used in an implemented calculation model to determine correction values. To determine the workpiece mass and the workpiece center of gravity, a measuring cycle is used that evaluates the motor current of the respective drive motors. To determine the workpiece mass and the workpiece center of gravity, the measuring cycle records the motor current of the servo motor for the swivel or rotary axis with a loaded workpiece-supporting assembly in various gravity-loaded swivel holding positions.

[0004] The magnitude of the unbalance says nothing about the workpiece weight. However, there is a desire for static compensation of machine deformations, which is not possible with knowledge of the unbalance alone.

[0005] Based on this, it is the object of the invention to provide a concept that enables the static weight compensation of workpieces on machine tools for unknown workpieces, whereby workpieces are also to be understood as the units possibly formed from the workpiece and holding device as well as the workpiece pallet.

[0006] This object is achieved with the machine tool according to claim 1 and with the method according to claim 8: The machine tool according to the invention comprises a workpiece holding device that is mounted on a machine frame for movement in at least one direction. The workpiece holder is configured to hold the workpiece and, if applicable, the unit formed by the workpiece and holding device (if present) as well as the workpiece pallet (if present). The term "workpiece" in the following description includes such units.

[0007] A drive device is provided for moving the workpiece holding device, empty or with the workpiece thereon, in a controlled manner in the direction of this one degree of freedom along a predetermined, preferably linear path. A detection device is configured to generate a signal that characterizes the force exerted by the drive device on the workpiece holding device. This detection device can be a separate force sensor or, as is preferred, a device that detects the drive force applied by the drive device, for example, based on the motor current of a driving electric motor.

[0008] The drive device typically also has a position detection device in order to be able to detect the current position of the workpiece holding device. From the change in position, the control device can also derive the current speed of the workpiece holding device's movement and from the change in speed, the acceleration of the workpiece holding device. The machine tool has an evaluation device to which the signal is fed that characterizes the force exerted on the workpiece holding device for driving purposes. In addition, the evaluation device has at least one signal that characterizes the position of the workpiece holding device and / or the speed and / or the acceleration thereof. The evaluation device is designed to calculate the mass of the workpiece holding device with workpiece from the exerted drive force and the current acceleration.The weight of the workpiece or its mass m is calculated as the difference between the mass m determined from the acceleration and the driving force and the known or previously measured empty mass m0 of the workpiece holding device (mass without workpiece).

[0009] The workpiece weight or its mass m can be determined as a separate calibration process, e.g. before workpiece machining begins or during a machining break. However, this mass can also be determined using movements that are carried out anyway during the machining process. For example, the workpiece holding device can be moved into a setup position to accept a workpiece. When transferring to the machining position, at least one acceleration process and at least one braking process (which is an accelerated movement with negative acceleration) must be carried out. Both movement phases (acceleration and braking) can be used to determine the workpiece weight, which is equal to the mass m of the workpiece. The workpiece weight can, for example, be determined as the average value of the workpiece masses m determined during the acceleration phase and the braking phase.

[0010] According to this concept, the machine tool determines the workpiece mass m before actually machining the blank. This allows any elastic machine deformations resulting from the workpiece weight to be taken into account right at the beginning of machining. For example, spring compliance with a known machine elasticity can be used to adapt the machine tool's machining program.

[0011] It is also possible to determine the workpiece mass m at a different time, for example, after rough machining operations (roughing), before fine machining operations, or before finishing operations. This has the advantage that machine deformations can be precisely taken into account at the precise moment when the final accuracy to be achieved on the workpiece is actually important.

[0012] It is also possible to determine the workpiece mass twice or more during machining, for example during short machining breaks.

[0013] In a sophisticated machine tool, not only the mass of the workpiece is determined as a numerical value, but also the position of the workpiece's center of gravity. The workpiece holding device of the machine tool has several degrees of freedom, each of which is assigned a drive device. In addition to the above-explained first degree of freedom, which preferably allows a linear movement, in particular a horizontal linear movement, there is at least a second degree of freedom that allows a rotational movement about at least a first axis of rotation A. This axis can, for example, be oriented parallel to the workpiece clamping surface. Furthermore, it can extend transversely to the direction Z predetermined by the first degree of freedom. This first axis of rotation A is preferably oriented horizontally.A test rotation of the workpiece around this first rotational axis A causes changing loads on the rotary drive of the rotational axis A if the workpiece's center of gravity is eccentric to the rotational axis A. If the workpiece's mass is known (because it has been previously determined), the evaluation device uses the maximum and minimum torque values ​​to determine the distance of the center of gravity from the rotational axis A (or the height of the center of gravity above the workpiece clamping surface) and the orientation relative to the rotational axis. Relative to the machine coordinate system, it determines the Y coordinate Y. s of the focus.

[0014] In addition, a test rotation of the workpiece around the first rotational axis A generates force signals in the drive unit for the first (linear) direction of movement. This drive unit holds the workpiece holding device in a specified (Z) position, with the control signals (or, for example, the motor currents) occurring in the position control loop being indicators of the forces occurring. The rotation test (rotation around the first rotational axis) can be used to determine the distance of the center of gravity from the rotational axis (or the height of the center of gravity above the workpiece clamping surface). Relative to the machine coordinate system, the evaluation unit determines the Y coordinate Y. s of the focus.

[0015] The workpiece holding device preferably has a further degree of freedom for a further axis of rotation B, which is preferably perpendicular to the workpiece clamping surface and thus oriented transversely to the first axis of rotation. To further determine the position of the center of gravity P, the axis of rotation B oriented perpendicular to the workpiece holding surface is preferably set parallel to the direction predetermined by the first degree of freedom, so that this second axis of rotation B is oriented horizontally. A test-based rotary movement about the axis of rotation B leads to a fluctuating torque on the rotary drive assigned to the axis of rotation B if the center of gravity P of the workpiece is eccentric to the axis of rotation B. This fluctuating torque is detected by the evaluation device.If the mass of the workpiece is known, because it has been determined beforehand, the evaluation device determines the distance of the center of gravity from the rotational axis B and the orientation to the rotational axis B from the torque maximum and the torque minimum. The evaluation device determines the X coordinate X in relation to the machine coordinate system. s and the Z coordinate Z s of the focus.

[0016] The test rotational movement around the rotation axis B also causes force signals S z of the drive device for the first degree of freedom Z, which result from the drive device's effort to hold the workpiece holder in a predetermined position in the first direction (Z). From the rotational position signals of the second rotation axis B or its drive and the force signals S zFor the first direction Z, the position of the center of gravity can be determined with respect to its angular orientation around the rotation axis B and the distance from it. In relation to the machine coordinate system, the evaluation device can use this to determine the X-coordinate X s and the Z coordinate Z s determine the center of gravity.

[0017] The method according to the invention uses the concept of test linear acceleration and / or deceleration to determine the workpiece mass. The resulting force F z , which is determined by the signal S zdescribed, together with the correspondingly determined acceleration z'' characterizes the mass of the workpiece. In principle, the time course of the acceleration z'' is not important. For example, it can be constant. However, it is also possible to choose a different time course of the acceleration z'', for example a sinusoidal time course. This method has numerical advantages because the position signal z multiplied by -1 can replace the double derivative of a position signal supplied by a displacement sensor.

[0018] The method further comprises rotating the workpiece about at least one of the rotational axes A or B at a constant speed or angular velocity. This causes a changing drive torque at the respective drive when the center of gravity is eccentric. From the magnitude and phase position of the drive torque, the evaluation device determines the position, ie, the coordinates X s, Y s , Z s of the center of gravity P.

[0019] To determine the coordinates X s , Y s , Z s of the center of gravity P, a sinusoidal force signal can also be used in the actuating device for the workpiece holding device for the first degree of freedom Z. The phase position of this sinusoidal force curve characterizes the angular position of the center of gravity P in relation to the respective axis of rotation A or BZB. By coordinate transformation, the Cartesian coordinates X s , Y s , Z s Calculate the center of gravity relative to a desired Cartesian coordinate system. The desired coordinate system can, for example, be related to the workpiece clamping location or the machine frame.

[0020] Further details of advantageous embodiments of the invention are the subject of the description, the drawings, or the claims. They show: Fig. 1 The machine tool according to the invention, in a schematic side view, Fig. 2 the machine tool after Fig. 1, in a schematic plan view, Fig. 3 a workpiece carried by the machine tool in a schematic side view, Fig. 4 and Fig. 5 different acceleration scenarios for determining the workpiece weight as a diagram, Fig. 6 the determination of a first coordinate of the shear point of the workpiece by rotation around the A-axis and observation of the reaction forces in the Z-direction as a diagram and Fig. 7 the determination of further coordinates of the center of gravity of the workpiece by rotation around the B-axis and the observation of the reaction forces in the Z-direction as a diagram.

[0021] Fig. Figure 1 illustrates a machine tool 10 according to the invention for machining, preferably by cutting, a workpiece 11 symbolically illustrated therein. This workpiece 11 is supported by a workpiece holding device 12, by means of which the workpiece 11 can be transferred into desired positions and held therein. The workpiece holding device 12 is supported by a machine frame 13 and is mounted thereon, for example, by means of a linear guide 14 ( Fig. 2) is movable in a predetermined direction, for example, the horizontal Z-direction, according to a first degree of freedom. The machine frame 13 rests on a support surface and is preferably mounted there in a stationary manner. The load emanating from the workpiece positioning device 12 and the workpiece 11 arranged thereon is transferred to the support surface via the machine frame 13. This can lead to elastic deformations on the machine frame 13 and the workpiece holding device 12. In order to compensate for these, the machine tool 10 is equipped with a load detection device 15, which is used to determine the mass m of the workpiece 11 and, at least in sophisticated embodiments, also to determine the position of the center of gravity P of the workpiece 11 ( Fig. 3) serves.

[0022] The load sensing device 15 uses a drive device 16 for load sensing, which is arranged on or in the machine frame 13 and moves and positions the workpiece holding device 12 in the Z direction according to the specifications of a control device 17. The drive device 16 can be a linear direct drive, a spindle drive with a servo motor, or other servo motor-gear arrangements. Such drive devices usually have position sensing means (not further illustrated) for continuously determining the Z position of the workpiece holding device 12. In addition, the drive device 16 typically includes a unit for determining the force exerted on the workpiece holding device 12. This unit can, for example, be the force based on the motor current required to achieve the movement of the workpiece holding device 12. It generates a corresponding signal S z, which is transferred to the control unit 17. The control unit 17, in turn, controls the drive device 16 to specify certain positions or movements.

[0023] The workpiece holding device 12, in turn, has at least a second degree of freedom for moving the workpiece 11, wherein this second degree of freedom can be a rotational movement about the rotational axis A. The rotational axis A is preferably oriented horizontally and lies transversely to the Z-direction. A drive device 18 is assigned to the rotational axis A ( Fig. 2), by means of which a workpiece carrier 19 can be specifically rotated about the rotational axis A and positioned in predetermined rotational positions. The drive device 18 communicates with the control device 17, from which it receives positioning commands and to which it provides position feedback and / or feedback of the torque (load torque) exerted by the workpiece carrier 19 on the drive device.

[0024] Furthermore, the workpiece carrier 19 can contain a turntable or a similar rotating device, by means of which the workpiece 11 can be rotated about a second rotational axis B. For this purpose, a drive device 20 can be provided, which communicates with the control device 17 as described in connection with the drive device 18. The drive device 20 is connected to the control device 17, from which it receives positioning commands and to which it provides position feedback and / or feedback of the torque (load torque) exerted by the workpiece carrier 19 on the drive device 20.

[0025] In modified embodiments of the machine tool 10, the workpiece holding device 12 can also be arranged to be movable in other axes or directions.

[0026] Furthermore, a machining unit 21 is mounted on the machine frame 13 for movement in at least one, preferably two, directions, for example, in the horizontal, transverse X-direction and in the vertical Y-direction. Further degrees of freedom can be provided to enable movement of the work spindle 22 provided on the machining unit. The latter is equipped with a rotary drive and configured to accommodate tools for machining the workpiece 11.

[0027] The control of the tool machining unit 21, in particular with regard to the movement in the X-direction and / or in the Y-direction, is subject to the control device 17, which executes a corresponding machine control program for this purpose and effects the desired machining through coordinated movement of the machining unit 21 in the X- and Y-directions, as well as through movement of the workpiece holder 12 in the Z-direction and targeted positioning about the rotation axes A and B. The control device 17 can be configured to modify the positioning commands for the machining unit 21 and / or the workpiece holder 12, which are related to an ideally rigid machine tool 10, such that an elastic deformation of the machine bed 13 and / or the workpiece holder 12 is compensated by the weight of the workpiece 11.If the workpiece carrier 19 bends slightly downward, for example, due to the weight of the workpiece 11 (which can be several hundred kilograms), and a hole is to be drilled at a desired location on the workpiece 11, the Y-position of the work spindle 22 must be corrected by the corresponding amount of deflection during the drilling process. To calculate this amount of correction, the control device 17 contains a hardware and / or software module 23 configured to determine the mass m of the workpiece 11. For this purpose, the signal S is sent to the evaluation device 23. z which has been generated by the detection device 15 and the force F exerted in the Z direction by the drive device 16 when accelerating the workpiece holding device 12 z indicates.

[0028] The machine tool 10 operates as follows: Before the start of machining, during machining breaks or shortly before the end of machining, the machine tool 10 carries out a test routine in which it first determines the mass m of the workpiece 11 and subsequently also the position of its center of gravity P.

[0029] To determine the mass, the workpiece carrier 12 is moved with linear acceleration. Fig. Figure 4 illustrates an exemplary movement sequence in which the workpiece carrier 12 is initially accelerated for a given first period of time, beginning at time t1 and ending at time t2, with a constant acceleration z''. The speed z' increases continuously. The resulting acceleration force F zis recorded and transmitted to the evaluation device 23. The control device 17 also has the acceleration z''. This is either predetermined or it is determined from the change in position z. From the acceleration z'' and the signal S z signaled force F z First, the total mass of the workpiece holding device 12 and the workpiece 11 is determined. The control device 17 subtracts the known mass of the workpiece holding device from this value and thus obtains the mass m of the workpiece 11 and thus its weight.

[0030] The mass determination with constant acceleration can also be carried out when braking the workpiece carrier 12, as in Fig. 4 right between times t2 and t3. Again, a signal S z which, for negative acceleration, is a measure of the force F acting in the Z direction zand from which the mass m of the workpiece can be calculated.

[0031] The mass m of the workpiece 11 can also be calculated by taking several measurements, for example, an acceleration and a deceleration of the workpiece carrier 12. It is also possible to provide a short phase of constant movement speed between the acceleration phase and the deceleration phase in order to determine the force F required to maintain a movement at a constant speed during this movement phase. z from the force required for acceleration (or added to the force required for braking) to improve the accuracy of the measurement. If the force required for movement at a constant speed, such as that required to overcome all frictional resistance, is known, such an intermediate measurement can be omitted and the acceleration measurement can still be corrected using this force component.

[0032] A different measurement strategy is in Fig. 5. There, the workpiece carrier 12 is moved back and forth in a sinusoidal oscillation in the Z direction. The resulting sinusoidal curve of the deflection z results in a likewise sinusoidal curve of the acceleration z''. This results in a sinusoidal signal F z , which characterizes the acceleration force. By dividing the instantaneous values ​​of the forces to S z proportional values ​​for the force F z Using the corresponding instantaneous value of the acceleration z'', values ​​for the mass m of the workpiece 11 are determined. These values ​​can be averaged to determine a smoothed value for the mass m of the workpiece 11.

[0033] Knowing the mass m of the workpiece 11, a first simple compensation for elastic deformations of the machine tool 10 or the machine bed 13 and / or the workpiece carrier 19 is possible. For more precise compensation, however, knowledge of the position of the center of gravity P is desirable. To determine the position of the center of gravity P, the machine tool 10 performs a further test sequence. There are several options for this: For example, the workpiece carrier 12 is held at a desired Z position, preferably at rest, by the drive device 16. In this phase, the control device 17 activates the drive device 18 to rotate the workpiece carrier 19 about the rotation axis A. Such a rotation is in Fig. 6 symbolized above. Since the center of gravity P lies outside the rotational axis A, an imbalance arises which, with uniform rotation around the rotational axis A, leads to a sinusoidal force curve, which the drive device 16 must apply in order to hold the workpiece holding device 12 at rest in the desired Z position. After the mass m of the workpiece 11 has been determined, the evaluation device 23 can use this force measurement to determine the distance of the center of gravity P from the rotational axis A. The greater the detected force F z according to the sinusoidal shape of the signal S z in Fig. 6, the greater the distance between the center of gravity P and the axis of rotation A for a given mass m. This applies for a known and fixed speed or angular velocity.

[0034] Determining the coordinate Y sof the center of gravity P by rotation around the axis of rotation A can also be determined without the aid of the force F measured in the Z direction zFor this purpose, the workpiece carrier 19 and the workpiece 11 located thereon are rotated at a constant Z position about the horizontal axis of rotation A. The axis of rotation B is at rest during this time. If the center of gravity P is eccentric to the axis of rotation A, the weight G due to the mass M of the workpiece 11 generates a sinusoidal load moment on the drive 18 of the axis of rotation A. The positive and negative maximum value (i.e. maximum and minimum) of this load moment occurs when the imaginary connecting line between the center of gravity P and the axis of rotation A is horizontal. From this maximum (and / or if desired also the minimum) of the load moment, for a given, i.e. previously determined mass m of the workpiece 11, the angle α at which the center of gravity P is in a polar coordinate system with the central axis A, and the distance of the shear point P from the axis of rotation A can be determined. Thus, ultimately, the coordinate Y sof the center of gravity P is known or determinable. The corresponding calculations are carried out by the evaluation device 23.

[0035] The distance between the center of gravity P and the axis of rotation A is as Fig. As shown in Figure 7 above, the position of the center of gravity P has not yet been clearly determined. It can lie at any point on the straight line 24 running at the same distance from the axis of rotation A. To more precisely determine the position of the center of gravity P, a second test is therefore carried out in which the workpiece 11 is rotated about the axis of rotation B. In one embodiment, the axis of rotation B is held perpendicular to the Z direction, i.e., it runs in the Y direction. In principle, however, it is also possible to work with an inclined axis of rotation B or with the B axis parallel to the Z direction.

[0036] By rotating the workpiece 11 around the vertical axis of rotation B, for example, a reaction force is generated in the Z direction, which, as previously described, leads to a force F z identifying signal S z When rotating at a constant angular velocity, this signal S z sinusoidal. The phase position of this sinusoidal signal curve, as in the test rotation around the rotation axis A, indicates the angle β at which the center of gravity P is located in a polar coordinate system with the rotation axis B as the reference axis. This clearly determines the position of the center of gravity P. If necessary, it can be converted into Cartesian coordinates X s , Z s of the workpiece 11, the workpiece carrier 19, or the machine tool 10. The corresponding calculations are carried out by the evaluation device 23.

[0037] Determining the coordinates X s and Z sThe center of gravity P can also be measured without detecting the force in the Z-direction. In this case, the B-axis is pivoted into a horizontal position (or into an inclined position) and the A-axis is locked to perform the measurement. During subsequent rotation of the workpiece 11 about the B-axis, a center of gravity P located eccentrically to the rotation axis B in turn causes a sinusoidal load moment on the drive device 20, from the magnitude of which, in particular from its maximum value and / or from its minimum value, the coordinates X s and Z s have it calculated.

[0038] With knowledge of the mass m and the position X s , Y s , Z sof the center of gravity P, the bending moment exerted by the workpiece 11 on the workpiece carrier 19 in each rotational position of the rotational axis A and thus the resulting deformation of the workpiece carrier 19 and, if applicable, other elements of the machine tool 10 can be calculated. The compensation of such deformations leads to an improved machining result. The evaluation device 23 and / or the control device 17 can be configured to influence the control commands for controlling the machining unit 21 such that a displacement of the workpiece 11 from its ideal position as a result of the deformation of the workpiece carrier 12 and / or the machine frame 13 leads to a corresponding adjustment of the machining unit 21 and thus to a compensation for a machining error that would otherwise occur.

[0039] The determination of the mass m as well as the position of the center of gravity P can be performed, in particular, after completion of roughing before performing fine machining. The material removal occurring during fine machining and, if applicable, finishing is typically so small that the resulting change in the mass m of the workpiece 11 and the change in the position of the center of gravity P no longer have a negative impact on the machining result.

[0040] The invention makes it possible to determine the exact spatial position of a center of mass P as well as the corresponding mass m of the workpiece 11 clamped on a rotary table. The exact spatial position of the center of mass means that its position X s , Y s and Z sin the Cartesian coordinates of a coordinate system whose zero point is formed by the intersection of the rotary table axis B with the support surface of the rotary table. The mass m is determined by acceleration in the Z direction and evaluation of the drive torque of the feed motor using a signal S z . Determining the coordinate Y s The center of gravity P is determined by rotation around the horizontal axis of rotation A and evaluation of the drive torque of the rotary drive 18. Alternatively, the coordinate Y is determined s by evaluating the drive torque of the drive device 16 or its feed motor. The determination of the coordinates X s and Z sof the center of gravity P is carried out by rotating the workpiece 11 around the horizontally positioned axis of rotation B and evaluating the drive torque of the rotary drive 20 or by rotating the workpiece 11 around the vertically positioned axis of rotation B and again evaluating the signal S z , which indicates the necessary holding force and thus the drive torque of the feed motor of the drive device 16. Reference symbol: 10 Machine tool 11 Workpiece 12 Workpiece holding device 13 Machine frame 14 Linear guide 15 Load detection device 16 Drive device 17 Control device 18 Drive device 19 workpiece carriers 20 Drive device 21 processing unit 22 work spindle 23 Evaluation device 24 straight

Claims

[1] Machine tool (10), in particular a machine tool with static compensation of the workpiece weight, with a machine frame (13) for stationary installation, with at least one workpiece receiving device (12) which is movably mounted with respect to the machine frame (13) with at least a first degree of freedom, with at least one drive device (16) which is connected to the workpiece receiving device (12) in order to move it in at least one direction (Z) determined by the first degree of freedom, with at least one detection device (15) which is designed to generate a signal (S z ) which counteracts the force (F z ) indicates with an evaluation device (23) to which the signal (S z ) in order to convert the signal (S z) and the movement carried out by the workpiece receiving device (12) to determine at least the mass (m) of the workpiece (11), characterized by , that the first degree of freedom allows a horizontal linear movement in order to move the workpiece holding device (12) linearly in at least one direction (Z) determined by the first degree of freedom, and that the executed movement is an accelerated movement, whereby the workpiece mass (m) is determined from the detected force (F z ) and the measured acceleration (z''). [2] Machine tool according to claim 1, characterized by that the workpiece holding device (12) is mounted so as to be movable in several degrees of freedom. [3] Machine tool according to claim 2, characterized by that each degree of freedom is assigned a drive device (16, 18, 20). [4] Machine tool according to one of the preceding claims, characterized bythat a second degree of freedom allows a rotational movement about at least one first axis of rotation (A) which is oriented transversely to the direction (z) predetermined by the first degree of freedom, and that the evaluation device (23) is designed to determine from the rotational movement about the axis of rotation (A) the distance of the center of gravity (P) of the workpiece (11) from the relevant axis of rotation (A) and / or the workpiece clamping surface and / or its Y-coordinate (Y s ) with respect to the direction (z) specified by the first degree of freedom. [5] Machine tool according to claim 4, characterized by that the evaluation device (23) for determining the Y-coordinate (Y s ) the signal (S z ) of the drive device (16) assigned to the first degree of freedom. [6] Machine tool according to claim 4 or 5, characterized bythat a further degree of freedom allows a rotational movement about at least one further axis of rotation (B) which is oriented transversely to the direction (z) predetermined by the first degree of freedom and transversely to the first axis of rotation (A), and that the evaluation device (23) is designed to determine the X and Z coordinates (X s , Z s ) with respect to the direction (z) specified by the first degree of freedom and a direction (x) transverse to the direction (z) specified by the first degree of freedom. [7] Machine tool according to claim 6, characterized by that the evaluation device (23) for determining the X and Z coordinates (X s , Z s ) the signal (S z ) of the drive device (16) assigned to the first degree of freedom. [8] Method for determining the mass of a workpiece (11) on a machine tool (10), comprising the following steps: accelerated movement of the workpiece (11) along a horizontal straight path in a first direction (z), Determining the force required for acceleration (F z ), Calculating the mass of the workpiece (m) based on the acceleration given to the workpiece (11) and the detected force (F z ). [9] A method according to claim 8, wherein: the workpiece (11) is rotated about a first axis of rotation (A), the torque (M A ) is recorded and from the torque (M A ) and the mass (m) of the workpiece (11), the distance of the center of gravity (P) from the workpiece clamping surface and / or a Y-coordinate (Y s ) of the center of gravity (P) transverse to the first direction (Z). [10] A method according to claim 8, wherein: the workpiece (11) is rotated about a first axis of rotation (A) which is oriented transversely to the first direction (z), the force (F) occurring in the first direction (z) z ) is detected, which is required to hold a workpiece holding device (12) at a fixed position in the first direction (z), from the power (F z ) and the mass (m) of the workpiece (11), the distance of the center of gravity (P) from the workpiece clamping surface and / or a Y-coordinate (Y s ) of the center of gravity (P) transverse to the first direction (Z). [11] A method according to any one of claims 8 to 10, wherein: the workpiece (11) is rotated about a second axis of rotation (B) which is oriented horizontally, the torque (M B ) is recorded and from the torque (M B) and the mass (m) of the workpiece (G) the position (X s , Z s ) of the center of gravity (P). [12] A method according to any one of claims 8 to 10, wherein: the workpiece (11) is rotated about a second axis of rotation (B) which is oriented transversely to the first direction (z), the workpiece (11) is rotated about the rotation axis (B) and the force (F z ) is detected, which is required to hold a workpiece holding device (12) at a fixed position in the first direction (z), and from the power (F z ) and the mass (m) of the workpiece (G) the position (X s , Z s ) of the center of gravity (P). [13] Method according to claim 11, characterized by that the second axis of rotation (B) is oriented vertically during the measurement.

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